In automobile parts, particularly in the parts made of resin used in an engine room, circumstances for their use are becoming severe as a result of temperature rise in an engine coolant and in the engine room, in accordance with increased engine performance and increased output. Further, in a cold district, a lot of anti-freezing agent for roads such as calcium chloride and zinc chloride is sprinkled as a snow-melting agent. Therefore, engine parts are also exposed to those chemicals as well. In the conventional Nylon 6 and Nylon 66, deterioration of the resin progresses under such severe using environments. Accordingly, several improvements have been attempted therefor.
Examples thereof are a method wherein fine glass fiber having specific fineness processed with a specific surface treating agent is used as a reinforcing material for Nylon 66 (Patent Document 1), a method wherein a high-melting polyamide having a melting point of 300° C. or higher (such as Nylon 6T type) is compounded (for example, Patent Documents 2 and 3) and a method wherein a resin of polypropylene type is compounded with Nylon 66 (Patent Document 4). However, any of those methods does not satisfy all of moldability, fluidity, weldability, low water absorbability and durability. Accordingly, there is yet room for improvement therein.
A polyamide resin has excellent characteristics such as mechanical characteristics, thermal aging resistance, and moldability. Accordingly, it has been widely utilized in various parts such as automobile parts, electric and electronic parts and industrial machine parts. However, deterioration by an action of heat and light is unavoidable for the polyamide resin. As to a method for enhancing the thermal aging resistance of the polyamide resin, methods wherein copper halide, potassium halide, oxazole compound or the like is added as a thermostabilizer have been known from old time already (for example, Patent Document 5).
As a result of these arts, the polyamide resin is used in such parts which are exposed to high-temperature environments of about 140° C. during use, in the fields of the automobile parts and the electric and electronic parts.
However, with regard to the engine room of automobiles for example, environmental temperature in the engine room has become high as a result of increase in engine output, tendency of arranging the parts with high density, etc. in recent years. Accordingly, there has been a demand for high level of thermal aging resistance which has not been aimed until now.
In order to fulfill this demand, there have been proposed a method wherein polyamide is compounded with finely granulated element iron (Patent Document 6), a method wherein polyamide is compounded with finely granulated and dispersed metal powder (Patent Document 7), a method wherein a mixture of two kinds of polyamides having different melting points from each other is compounded with a copper compound and iron oxide (Patent Document 8), a method wherein polyamide is compounded with a thermostabilizer such as copper iodide or potassium iodide and a composite oxide such as tri-iron tetraoxide (containing iron (II) oxide) (Patent Document 9), etc. It has been said that products obtained by these methods are excellent in thermal aging resistance even under high-temperature environments of about 200° C.
However, in the methods of Patent Documents 6 and 7, there is a danger of ignition during preparation of a composition whereby the preparation is not easy. In the method of Patent Document 8, there is a disadvantage that the effect is achieved only by a very limited composition. In the method of Patent Document 9, there may be a case wherein stability and reproducibility of the thermal aging resistance and the mechanical strength are inferior. As such, any of them is to be still improved.